Many applications require that a higher level of current and power to be delivered to a load. At the same time, modem electronic devices require small, low cost, high density power converters. The paralleling of power converters provides a way for two or more individual, small, high density power converter modules to be coupled in parallel so as to supply the required power for high current loads and to provide redundancy. It is desirable that the individual converters in the parallel configuration share the load current equally in a stable, regulated manner. Furthermore, better current sharing between the converters reduces power converter stress which increases the reliability of the paralleled converter system.
Theoretically, where two power converter modules are connected in parallel, for example, they will have current sharing levels of fifty percent each. These levels assume that relevant parameters, e.g., resistor, capacitor and inductance values, are the same for each module. In practice, due to device tolerances, etc., such an assumption is not warranted. As a result, each of the two converter modules will have different current sharing levels. It has been experimentally shown that, for conditions close to full load, paralleled converter modules typically can expect to have 1%-tolerance resistors, 1%-tolerance Pulse Width Modulation (PWM) generators, and 20% tolerance inductors. As a result, respective current sharing levels of 40% and 60% are the best to be expected in practice. A need exists for reducing this difference in current sharing levels in a low cost way.
Efficient current sharing requires a means for measuring the current. Known circuits for current sharing for parallel buck converters, for example, utilize a sense resistor in series with the output inductor for each of the paralleled converters. As illustrated in FIG. 1 and as is well known, a basic buck regulator comprises a switch 10, a diode 12, an inductor 14, and a capacitor 16, connected in a conventional way between an input terminal to which is coupled an input voltage Vin relative to ground, and an output terminal at which the buck regulator generates a regulated output voltage Vout relative to ground. The switch 10 is typically a power MOSFET which is controlled in a known manner by a control circuit, e.g., a pulse width modulator (not shown) that is responsive to the output voltage Vout. When the switch 10 is closed, the capacitor 16 is charged via switch 10 and inductor 14 from the input voltage Vin to produce the output voltage Vout, which is consequently less than the peak input voltage Vin. When switch 10 is open, current through the inductor 14 is maintained via diode 12. Resistor 18 is a sense resistor connected in series between inductor 14 and the output. For current measurement, the voltage drop across the sense resistor is measured. The sense resistor must have sufficient resistance to provide a voltage that can be sensed accurately. A drawback of converter circuits that use a sense resistor is that significant power is lost in the sense resistors when the converters are providing high output currents, thereby reducing the efficiency of the converters.
In another method of sensing output current for a buck converter, current is sensed using the voltage drop across the inductor. One known example of inductor sensing is disclosed in U.S. Pat. No. 6,424,129 in which a resistor and a capacitor are connected in parallel with the output inductor. This patent has the drawback of not providing any active current sharing to enable the current levels output by paralleled converters to be adjusted.
Another method of sensing output current for a buck converter is MOSFET sensing, wherein the drain-source voltage of the MOSFET is measured when the MOSFET is switched on. The accuracy of the sensed measurement is dependent on the characteristics of the MOSFET which vary from device to device. The drain-source on resistance typically has a large tolerance that varies from device to device. The drain-source on resistance for MOSFET devices also varies with temperature and this variation is often not well defined.
A need therefore exists for a circuit that actively and efficiently controls the current output by respective power converters in a system having paralleled power converters. There is also a need for a circuit that provides this function using a lossless sensor having fewer and lower cost components.